Driven by a range of political passions — including anticapitalism, environmentalism, and gay and indigenous rights — the diverse collection of anarchists, communists and socialists has found common cause in opposing right-wing extremists and white supremacists. In the fight against the far right, antifa has allied itself at times with local clergy, members of the Black Lives Matter movement and grass-roots social-justice activists. It has also supported niche groups like Black Bloc fighters, who scrapped with right-wing forces in Berkeley this year, and By Any Means Necessary, a coalition formed more than two decades ago to protest California’s ban on affirmative action for universities.